They're all male bipedal humanoids, and the vault animation is the same for every class.
What's to stop them applying the same animation to the larger bodies? Would be easy enough. Yes, it would look silly, but it shouldn't be a problem.
Exactly.
It's just as easy to apply the animations to the Heavy body types as it is to the others. Only thing is that they have more girth, hence why it would look silly, and they seem to have different skeletal structures which may not react to the animations as fluidly as Light/Med. Hence why it would be so noticeable.
If movement other than walking broke your disguise there wouldn't be much point in being disguised in the first place. Having a teammate who doesn't nothing but slowly walk to wherever he's going would be more than a bit suspicious.